Certain types of pressure sensors are designated as “leadless” sensors. The designation “leadless” arises from the fact that these pressure transducers are fabricated such that they can make contact with a header by means of internal contacts instead of typical wire leads. An example of such a leadless pressure sensor is depicted in U.S. Pat. No. 5,955,771 issued on Sep. 21, 1999 entitled “Sensor for Use in High Vibrational Applications and Methods for Fabricating the Same” to A. D. Kurtz et al. Such leadless pressure sensors as depicted in the above noted patent are capable of operating in many harsh environments and basically are moisture resistant. These capabilities are attributed to having only the micro machined side of the sensor, containing no active elements, exposed to the operating environment. This side, being comprised substantially only of silicon can thus withstand numerous harsh environmental conditions, including high temperature, corrosive, oxidizing, and conductive media. The other side of the sensor containing the piezoresistive network is hermetically sealed within a vacuum cavity. FIG. 1 depicts an absolute leadless transducer assembly according to the prior art. As one can ascertain from FIG. 1 the sensor chip 10 is located in such a manner that a pressure P is applied to the backside of the diaphragm. The pressure or piezoresistive elements 16 are contained within a vacuum cavity 12 which is formed between the sensing chip 10 and the glass wafer 17. The entire assembly as shown is coupled to a glass header 13 where header pins 14 and 15 make contact with the contacts of the sensing chip 10. The piezoresistive sensors 16 are located within the vacuum cavity 12 and a pressure is applied to the rear side or back of the diaphragm as that side opposite to the side containing the piezoresistor 16.
FIG. 2 illustrates another prior art pressure sensor operative as a differential sensor. The same reference numerals have been utilized in FIG. 2 as in FIG. 1 to denote corresponding elements. Essentially the sensor chip 10 includes a silicon wafer which is bonded to a glass wafer 17. In the case of the unit in FIG. 2 it is seen that a first pressure is applied to the diaphragm side of the sensing element designated as P1. A through hole 18 accommodates a reference tube 19 allowing a pressure P2 to be applied to the other side of the sensor element 10. This side of the sensor element contains the piezoresistive elements 16. Thus, in the embodiment shown in FIG. 2 the piezoresistive elements 16 are directly exposed to the environment in which the pressure P2 is present. It is of course understood that such a differential device as depicted in FIG. 2 would utilize the harsh environment associated with P1 to be exposed to the underside of the diaphragm and hence to be removed from the piezoresistive sensors 16. However in order to obtain a true differential operation of the sensor 10 and to obtain complete environmental protection both sides of the sensor 10 should be protected from the environment.